Fixed frequency oscillator



June 17, 1958 A. KELLY 2,839,685

' FIXED FREQUENCY osCILLAToR Filed May 1s, 1955 [/v new rola D04/fa@ 6l Knuf 2,839,685 W FIXED FREQUENCY osCILLAToR Dunford A. Kelly, Los Angeles, Calif., assignor, by direct and mesne assignments, to Harvey O. Riggs and International Electronic Research Corporation, both of Burbank, Calif., as joint owners Application May 13, 195s, serialNo. 508,167

v7 claims. (cl. 25o-36) This invention relates to electrical oscillators and, more particularly, to an oscillator 4which will provide a substantial power output at closely controlled frequency and amplitude with low harmonic distortion andlow internal impedance.

High power output and low distortion and good frequency stability are conflicting requirements in oscillator design. The `co-nventional manner of improving harmon-4 ic distortion and frequency stability is to reduce the power output to a minimum and `supply the power require ments by use of separate amplifiers. This results in a decrease in efficiency and an increase in the complexity of the overeall setup.

It is an object of the invention to provide an oscillator having high power output and low harmonic distortion and low internal impedance; for example, an oscillator having an output of 3 watts with a total harmonicdistortion of less than 1/20 of 1 percent and an internal im* pedance of J/20 of 1% of the load impedance. v v

A further objectof the invention is to provide Ysuch an oscillator in which line voltage changes, load changes, tube aging, tube replacement, temperature and humidity changes 4and mechanical shock and vibration have neglif gible effect on the above characteristics. v n* yIt is a further object of the invention to provide anios; cillator having high frequency land amplitude stability 'United StatesPatent 2,839,685 Patented June 17, 1958 fectively regulates the circulating energy. The bridge output is coupled to a tuned circuit through a resistor of high enough resistance to avoid undue loading of the ytuned circuit. Such an oscillator suffers severely from the fact that the impedance of a lamp bulb 'bridge is inherently low, with the consequence that the voltage output from the bridge is unduly low. After being further reduced by the voltage dividing action of the resistor and tuned circuit, very little signal voltage is supplied to the amplifier.

The foregoing condition severely limits the negative feedback which can be introduced for stabilization. I-t can be readily seen that the greater the voltage from the tuned circuit, the greater they negative feed-back voltage that can be used. The amplier requires a certain fixed net signal voltage. The posi-tive feed-back voltage minus themegative feed-back voltage is the net signal voltage. VIt follows that the greaterV the positive feedback voltage, the greater can be `the negative feed-back voltage, and, of course, the greater the negative feed-back voltage, .the greater the stabilization o-f the oscillator.

`and one in which the output frequencyvr and amplitude may be adjusted when desirable.

Another object of the invention is to provide such an oscillator which can be operated without requiring supply voltage regulation or buffer amplification between the oscillator output-and theload. Y v y Basically, an oscillator isfan amplifier in which part ofthe output is coupled backto the input in'proper phase and sufficient amplitude so that any signal presentwill build up until further increase is vprevented by some limitingaction. A frequency determining element, such as a tuned circuit, a crystal,.or arnechanical resonator, is inserted between the amplifier output and the amplifier input` and controls the Vfrequency of the oscillations by making the amount, of thesignal coupled to theinput a maximum "at a particular frequency. The limiting action or'arnplitude -control of an oscillator is often provided by a bridge networkwhaving a non-linear impedance in one of its arms, and khaving anL output vwhich is an inverse function of its input, or by other known voltage limiting devices. That is, when the amplitude output of v the-amplifier increases, the transmission of the amplitude control device decreases, thereby reducing the signal cou -features of the invention is shown in Fig. 1.

A more particular object of the invention is the provision of au oscillator of the class described, in which the positive feed-back voltage is very greatly increased, with negative feed-back voltage correspondinglyA increased, e. g., of the order of 40 to 50 decibels or higher, yielding an oscillator of vastly improved stabilitylk According to ythe preferred practiceV of the invention, a voltage step-up transformer is incorporated between a lamp 'bulb bridge network and a parallel LC tuned circuit, and -is made a part of this tuned circuit. Thus, a primary transformer winding may 4be placed on the tuning coil, and in a typical case, a 'turns ratio of l tolOOO may 'be used, giving va voltage Ygain of 1000 to 1. In addition, the transformer furnishes a good impedance match between the low lmpedance lamp bulb bridge andthe high impedance tuned circuit, giving a high improvement in signal voltage'across the tuned circuit. With such increase in positive feed-'back voltage fed to the vamplifier, the negative feed-back voltage canbe, andfis, coinmensurately increase-d. Verygreat voltage and frequency stability are thus gained.` v H l i The invention also comprises novel details of construction and novel combinations and arrangements of parts, which will more fully appear in the course of the following description. However, the drawing merely shows and the description merely ldescribes the preferred emb'odiments of the presentinvention las `applied to an osoillator, which are given by way of illustration or example only.

'In the drawings:

Fig. 1 is a block diagram of one embodiment of the invention; f .l Y Y g v.

kFig. 2 is a circuit diagram of an exemplary instrumentation of the embodiment of Fig. 1; and

n Fig. 3 shows a modification of the circiutof Fig. 2.

A block diagram of an oscillator incorporating the Therein, numeral 10 is a conventional amplifier having output terminals 1,1, 12. An amplitude control unitv 13 and a frequency control unit 14 are connected to the input 15 of the amplifier 10. It is preferred inthe practice of the invention to have the output of the amplitude control uni-t 13 connected to the frequency control unit 14 and then to have the output of the frequency control unit connected to the input 15 of the amplifier. ative feedaback path 16 couples the output of the amplifier 10 to the input 15. rDhe output of the amplifier is also .connected to the amplitude control unit 13 by a positive feed-back circuit means A17.

is illustrated in Fig. 2. The output of the amplifier 10 A degenerass-9,685

is coupled back to the amplitude control unit 13 through a resistor 2i) and lead 1'7, furnishingv positive feed-back. The amplitude control unit 13 consists of a bridge circuit havingva resistor 21, a nonlinear impedance element 22, suchas the' indicated lamp, and a tapped primaryv winding 23 of a step-up voltage transformer 24 as the' arms thereof'. The tap of the winding' 23 is-connectedk to circuit ground, as is the output terminal 12 of the amplifier. A secondary windingl 25 of the transformer 24 is tuned to the desired frequency of' oscillation by a shunt capacitor 26, the output of this inductance-capacitance tuned circuit being coupled to the amplifier input 15.

A voltage dividing network consisting of serially connected resistors 3f), 3-1- is connected' across the' amplifier output. The junction point of the resistors 3%, 31 is connected` to thel input of the amplifier through the tuned secondary winding 25 and capacitor 26 by' a connection 32; This connection provides the degenerative voltage feedback of the oscillator land the desired feedback ratio may be obtained by adjusting the relative values of the resistors 3i?, 31. A further'reduction to substantially zero in the terminal impedance of the oscillator may be obtained by inserting a resistor 33 in one of the output leads of the amplifier, thereby producing positive current feedback. Y

The bridge circuit comprising the amplitude control unit 13r is typically a low impedance device, being directly coupledA to the amplifier output. In order toV achieve high( stability and precise amplitude control', it is desirable thatthe operating point of the bridgev be maintained very close to balance. For thesel two reasons, the voltage output of the bridge is very small. Because of thissmall output, the bridge stabilized oscillators of the prior art have not been able to utilize large amounts of degenerative feedback.' The transformer 24 provides a large voltage stepup ratio between the ampli tude control unit andthe frequency control unit and amplifier input, thereby providing a high voltage gain between the bridge output and the amplifier input. in the embodiment of Fig. 2 a voltage gain of 1,()0Oz1 is typical and higher gains are possible. Then the feed back ratio provided by the resistors 30, 31 may be in the Vorder of 40 to 50 decibels. A

An oscillator utilizing the features illustrated in Fig. 2 can provide 3 watts of output power with an output regulation of /o of 1 percent. Higher power Voutputs may be obtained with4 similar performance figures. An oscillator having 100 watts output power can have 1A() of l percent regulation. y

The output of the amplitude control bridge 13 decreases as its input increases. The element` 22 is nonlinear, its impedance increasing with the current in it. The bridge is operated in an unbalanced condition. `lf sufficient input signal where coupled to the rcontrol bridge, the bridge would be balancedV and no signal would be transmitted to the amplifier input.' Therefore, the balance point is never reached and the amplitude of the oscillator stabilizes at a level bellow this balance point. When greater sensitivity is desired, a second nonlinear elementr may be substituted for the resistor 21.

In the preferred embodiment ofthe invention the nonlinear element 22 is a heated filament lampbulb or the connected to the primary winding 23 and the operation of the amplitude control unit is' the same as that of the embodiment of Fig. 2.

The amplifier ifi is conventional in design and may be constructed with vacuum tubes, transistors, or other similar devices. The stability of the over-all circuit is such that supply voltage regulation is not necessary in order to obtainf the characteristics referred to above. An oscillator embodying the features of the invention can provide a frequency stability of plus or minus 1/0 of a cycle at 1000 cycles per second with a supply voltage variation of plus orminus 10 percent. in embodiments of the invention utilizing a vacuum tube amplifier, it has been found that the amplitude and frequency stability of the circuit is such that tubes may be changed without requiring adjustments in the circuit and even in some cases that one type of tube may be substituted for another related type, such as a 6AKS for a 6AU6 without seriously affecting the magnitude or frequency of the oscillator output. Ordinarily, the differences in tube capacitances existing between' tubes and the varia` tions occurring with mechanical' shifting of the tube structure are great enough to measurably affect the capacitance of the frequency controlling tuned circuit. However, the provision of a very large amount of degenerative feedback at the amplifier input greatiy reduces the voltage coupled from 'the frequency control circuit to the amplifier and thereby greatly reduces the effect of capacitance charges in the vacuum tubes. in the oscillator of the invention, the capacity effect of a vacuum tube amplifier is negligible, resulting in the stable operation described.

When a vacuum tube amplifier is utilized, it is prefer-l able to have the input 15V connected to the control grid of the input tube. This produces an improvement in performance, particularly'in the form of distortion reduction, over the'conventional method of coupling the degenerative feedback to the cathode and' the oscillating signal to the control grid respectively of the input tube.

When low distortion is desired in an oscillator, it is conventional to operate the amplifier portion at a small fraction of its maximum signal level. Such practice results in a drastically lowered over-all efficiency for the oscillator and very low power output. A particular advantage of the oscillator of the invention is that the above given performance figures are obtained when the amplieris being operatedl at its rated maximum output.

Although several exemplary embodiments of the invention have been disclosed and discussed, it Will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention. Y

I claim as my invention:

l. In an oscillator, the combination of: an amplifier having an input and an output; degenerative feedback means coupling said amplifier output to said lamplifier input; a low impedance control bridge having; an input and an output, at least one of the arms of said bridge including a nonlinear elementV vthe impedance of which varies with the current in it; positive feedback circuit means connecting saidv amplifier output to said bridge input; frequency control means tuned to a predetermined frequency and having an output connected to said amplifier input; and voltage step-up transformer coupling said bridge output to said frequency control means, said transformer having a low impedance input and a high Therein the bridge circuit I impedance output. Y

2. In an oscillator, the combination of: an amplifier having an input and an output; degenerative vfeedback means coupling said amplifier output to said amplifier input; a low impedance control bridge having an input and an output, at least one of the arms of said bridge including a nonlinear element the impedance of which varies with the current in it; positive feedback circuit means connecting said amplifier output to said bridge input; frequency control means tuned to a predetermined frequency and having an output connected to said amplifier input; and voltage step-up transformer having a low impedance primary connected to said bridge output and a high impedance secondary connected to said amplifier input, said high impedance secondary being tuned to a predetermined frequency.

3. In an oscillator, the combination of: an amplifier having a high impedance input side and a lower impedance output side, a low impedance voltage amplitude limiting device comprising a network including at least two bridge arms having a common junction, a non-linear impedance element whose impedance varies with current flow therethrough included in at least one of said bridge arms, means for deriving a positive feedback voltage from the output side of said amplifier and impressing it on said bridge arms; a voltage step-up transformer having a primary winding connected to said bridge arms, a parallel resonance tuned circuit comprising the secondary winding of said transformer and a capacitor in parallel, said tuned circuit being coupled to the input side of said amplifier, and degenerative feedback circuit means coupling the output side of said amplier to the input side of said amplifier. v

4. In an oscillator, the combination of: an amplifier having a high impedance input side and a lower impedance output side, a low impedance voltage amplitude limiting Vdevice and a voltage step-up transformer, said limiting device comprising a pair of bridge arms having a common junction forming an input terminal, a nonlinear impedance element whose impedancel varies with current flow therethrough in at least one of said bridge arms, and a second pair of bridge arms comprised of a primary winding of said step-up transformer having a center tap providing a second input terminal, means for deriving a positive feedback voltage from the output side of said amplifier and impressing it across said input terminals, a parallel resonance tuned circuit comprising the secondary winding of said transformer and a capacitor in parallel, said tuned circuitbeing coupled to the input side of said amplifier, and degenerative feedback circuit means coupling the output side of said amplifier to the input side of said amplifier.

5. In an oscillator, the combination of: an amplifier having a high impedance input side and a lower impedance output side, a low impedance voltage amplitude limiting device comprising a bridge network including a first pair of bridge arms having a common junction forming an input terminal and having at the opposite ends thereof a pair of output terminals, and a second pair of bridge arms connected between said output terminals and a common junction forming a second input terminal, a non-linear impedance element whose imped ance varies with current flow therethrough in at least one of said bridge arms, means for deriving a positive feedback voltage from the output side of said amplifier and impressing it across said bridge input terminals, a voltage step-up transformer having a primary winding connected across said bridge output terminals, a parallel resonance tuned circuit comprising the secondary winding of said transformer and a capacitor in parallel, said tuned circuit being coupled to the input side of said amplifer, and degenerative feedback circuit means coupling the output side of said amplifier to the input side of said amplifier. l

6. Inan oscillator, the combination of: an amplier having a high impedance input side and a lower impedance output side, said amplifier utilizing a first stage amplifying device having a control grid; degenerative feedback circuit means coupling said output side to said control grid; a low impedance voltage amplitude limiting device; means for deriving a positive feedback voltage from the output side of said amplifier and impressing it on said amplitude limiting device; and coupling means including a voltage step-up device and a tuned circuit fed by the output of said amplitude limiting device and whose output is coupled to said control grid.

7. The subject matter of claim 3, wherein said degenerative feedback means couples the output side of the amplifier to the input side thereof through said tuned circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,275,452 Meacham Mar. 10, 1942 2,280,293 Kreer Apr. 2l, 1942 2,379,694 Edson July 3, 1945 2,611,874 Jacobsen et al. Sept. 23, 1952 FOREIGN PATENTS 599,963 Great Britain Mar. 24, 1948 

